This invention relates to a method and apparatus for coating metal in either sheet or coil form with a protective or decorative coating and for drying and curing the coating. The invention is particularly directed to a method and apparatus for drying and curing a liquid coating which has been applied to individual sheets of a metal substrate. The invention also includes a sheet produced by the process of the present invention.
Sheet metal which is to be utilized for producing various products, such as metal cans and ends and decorative metal pieces, may have a coating applied to the metal for protective or decorative purposes. The metal can be in coil form or in the form of individual sheets. The protective coating is usually applied to the metal in liquid form by various techniques well known to those skilled in the art, such as a roller coater, dipping, spraying and the like, as the metal substrate is passed through the coater. Various coatings and inks can be used which are well known to those skilled in the art, including for example, vinyls, epoxys, alkyds and phenolics. These coatings include various resins and pigments dissolved in a solvent. The solvent can be either a volatile organic solvent or may be an inorganic solvent, such as a water based solvent.
The present invention is primarily directed to coating individual sheets of ferromagnetic metal with or without tin or other metal coatings and having a gauge of approximately 0.004 to approximately 0.060 inches. The sheets may be rectangular and have a size, for example, of up to 54 inch by 56 inch. These dimensions are intended to be examples only and are not provided by way of limitation. The invention may also be applicable to similar gauge metal in coil form.
As will be appreciated by those having ordinary skill in the art, the normal practice is that liquid coating which has been applied to the metal substrate is dried and cured by the application of heat. The coating manufacturer usually specifies the temperature to which the coated metal must be heated and the duration for which the coated metal must be maintained at the specified temperature to achieve a proper cure of the coating.
Prior to the present invention, the most commonly used method of and apparatus for drying and curing the coating applied to metal in coil or sheet form was the use of a gas fired convection oven. The coated metal sheet or coil is baked by being slowly conveyed through the gas fired convection oven, whereby the metal sheet and coating are gradually heated to the desired temperature, maintained at that temperature for the specified duration. The oven may include a cooling zone to gradually reduce the temperature of the metal substrate to a point where it can be handled by appropriate material handling apparatus without damaging the protective or decorative coating. A normal cure cycle for organic coatings inks and solvents utilizing a conventional gas fired oven is two minutes to bring the metal up to cure temperature followed by maintaining the sheet at cure temperature for eight minutes to drive off the remaining solvent and provide the proper cross linking of the molecules to provide a cured coating.
In the case of individual sheets which have been coated, the convection oven typically includes a plurality of spaced apart wire wickets mounted on an endless conveyor chain. The coated metal sheets are transported to the convection oven where an individual wicket picks up an individual sheet of coated metal and conveys it in a generally vertical position through the convection oven. Hot gases generated in the natural gas convection oven circulate around the metal sheet to cure the coating. The wicket will discharge the sheet which has been dried and cured onto suitable material handling apparatus at the outlet of the oven.
In a convection oven, the coated metal substrate is heated from the outside causing a skin to be formed on the surface of the coating. This skin will serve to trap liquid solvents in the coating below this skin. In order to overcome this tendency, the coating manufacturer will add expensive and environmentally unfriendly retarding agents to the coating to prevent rapid cure of the coating surface prior to the release of solvents and product release compounds. These retarding agents not only add to the cost of the coatings, but also increase unwanted hydrocarbon emissions.
A further problem with utilizing convection ovens is that "wicket ghosting" can occur. The wire wickets which support the sheets in a vertical position are often preheated to insure proper cure of the coated sheet which is in contact with the wicket. When the cold sheet contacts the hot wicket, the heat drives the solvent and volatile products from the sheet on and around the wicket-sheet contact area. This condition can change the appearance and sometimes the color of the sheet. This produces a silhouette pattern in the shape of the wicket on the sheet. The resulting sheet may be unacceptable to the user and have to be scrapped. It is found that by utilizing the precuring process and apparatus of the present invention, the temperature differential between the wicket and the precured sheet can be kept to a minimum to substantially reduce or eliminate wicket ghosting.
A further disadvantage of the use of convection ovens is that the solvents which are fumed or volatilized by the heat from the convection oven tend to contaminate the conveying mechanism, burners, controls and exhaust duct of the convection oven through the formation of soot which may be generated when the solvents contact the open flame of the convection oven. Fires can result which may damage not only the convection oven, but also the coated sheets which are contained in the convection oven. In addition, the volatilized solvents must be captured or incinerated following the convection oven in order to comply with environmental requirements. With the use of a convection oven only, since the solvents are mixed with the products of combustion of the convection oven, they cannot be condensed and recycled.
Prior to the present invention, it was known to utilize electromagnetic induction coils for heating the metal substrate to cure the liquid coatings which have been applied to the metal in coil or sheet form. The use of an electromagnetic induction coil has the advantage that the metal is rapidly heated from the inside outwardly toward the surface of the coating. Heating is accomplished by passing the coated strip through or under an electromagnetic induction coil to produce eddy currents in the sheet metal to rapidly heat the metal. Because the coating is heated from the inside out, a skin is not formed on the surface of the coating and the volatilized solvents are allowed to escape through the still liquid surface of the coating. Some examples of prior apparatus and methods for coating metal strip in coil form are shown in U.S. Pat. Nos. 3,561,131 and 3,576,664, issued to Swartz, and U.S. Pat. Nos. 4,680,871 and 4,694,586, issued to Reznik, and U.S. Pat. No. 4,761,530, issued to Scherer et al. In many applications, the metal which has been heated in the induction coil is promptly cooled.
With the use of an induction coil, the solvents can be volatilized and then condensed in a condenser for further use. This reduces emissions to the atmosphere, thereby reducing environmental problems, and has the economic advantage of being able to recycle the solvents. Examples of prior patents which disclose condensing volatilized solvents include the aforesaid patents to Reznik and Swartz, as well as U.S. Pat. No. 4,370,357 to Swartz.
Induction curing is usually a rapid curing process and may not be suitable by itself for meeting the coating manufacturer's specifications for curing the coating. Further, total curing in an induction coil may not be energy efficient.
Prior to the present invention, induction heating has been usually applied to coiled materials, such as flat metal coil and wire, whereby the metal can be unwound from one coil, passed through the induction coil to heat the metal, and then immediately wound onto another coil. A conveyor mechanism need not be passed through or near the induction coil.
Sheets of material have been coated and cured in a process and apparatus described in U.S. Pat. No. 3,068,119, issued Dec. 11, 1962, to Gotsch. The patentee describes an increase in temperature at a rate of 200.degree. F. per second to achieve a temperature of between 500.degree. and 800.degree. F. by moving the coated sheet through the induction coil at a rate so that the coated sheet spends 2 to 5 seconds within the heating zone. The patentee then proposes to hold the coated sheet at the elevated temperature for a period of time. The patentee does disclose certain advantages of the use of an induction heating method and apparatus, but does not disclose details as to how to convey the individual sheets of material through the induction coil or how to prevent overheating of the metal substrate, particularly near the edges of the substrate.